The present invention relates to an image information reading system to read, for example, the radiation image information stored in a photo stimulable storage phosphor sheet and the image information recorded on an X-ray film and printing film. It also relates to a light-collecting device and light-collecting system used in said image information reading system.
Conventionally, the following image information reading system has been put into practical use: Beam such as laser beam is scanned two-dimensionally on the sheet where image information is recorded, and beam is applied to the sheet. The light containing the image information obtained therefrom is detected by a light detecting means such as a photomultiplier. The image information recorded on the sheet is read out by said image information reading system.
Said image information reading system includes a printing plate scanner, an X-ray film scanner and a radiation image reading system to read the radiation image information recorded on the photo stimulable storage phosphor sheet.
The following description mainly concentrates on the radiation image reading system using the photo stimulable storage phosphor sheet:
When radiation is applied, part of the energy of radiation is stored. If stimulating lay such as visible light is applied thereafter, the light according to stored energy is emitted by a photo stimulable storage phosphor. Using this photo stimulable storage phosphor, radiation image of a subject such as human body is photographed and recorded on the sheet-formed photo stimulable storage phosphor. The photo stimulable storage phosphor sheet is scanned by stimulating lays such as laser to obtain the light emitted by the stimulable phosphor, which is subjected to photoelectric reading to get an image signal. Based on this image signal, the image signal of the subject is displayed on the recording medium such as photosensitive material or CRT as a visible image. Such a system is put into practical use.
A radiation image reading system used in said system comprises    (1) a scanning optical system to apply stimulating lays to a photo stimulable storage phosphor sheet storing radiation image information,    (2) a photo-detector to detect the light emitted by the stimulable phosphor by application of stimulating lays, and    (3) an optical guide to lead the light emitted by the stimulable phosphor to said photo-detector.
In said system, the energy of the light emitted by the stimulable phosphor by application of stimulating lays is very weak and non-directional. The SN ratio of radiation image obtained from the reading system greatly depends on the energy received by the photo-detector. What is important in the design of this system is how to effectively lead this weak light emitted by the stimulable phosphor to the photo-detector. Various methods have been proposed by many companies.    (1) A means of improving the efficiency at which the light emitted by the stimulable phosphor is transmitted to the photo-detector (hereinafter referred to as “light-collecting efficiency”) is exemplified in Japanese Patent Laid-open NO.16667/1987, which discloses an image reading system. This system has a light receiving surface extending along the horizontal scanning line, and is characterized by having a long photomultiplier arranged close to the photo stimulable storage phosphor sheet, and a reflective optional member which reflects the light emitted by the photo stimulable storage phosphor sheet toward said light receiving surface.
In this case, it is possible to avoid reduction of light-collecting efficiency by generating a reflection enhancing film on said reflective optional member or by generating a reflection preventive film on the end face of a light guiding sheet-formed member such as a polymethyl methacrylate (PMMA) plate. Said light guiding sheet-formed member is of a flat plate, and all the incoming light emitted by the stimulable phosphor is basically reflected inside without leaking outside.
Further, the light receiving surface of a longer photomultiplier is sufficiently long in the horizontal scanning direction, and is sufficiently wide with respect to the ongoing surface of the light guiding sheet-formed member. So almost all the light, emitted by the stimulable phosphor and coming from the light guiding sheet, reaches the photo-detector. This ensures basically excellent light-collecting efficiency.    (2) U.S. Pat. No. 5,598,008, for example, discloses the radiation image reading system to read the radiation image information stored on the photo stimulable storage phosphor sheet, wherein an elongated and axially symmetry light-collecting device is coated internally with diffusion reflecting substance.
However, the prior art of said configuration has the following problems:    (1) The system disclosed in the Japanese Patent Laid-open NO.16667/1987 has a longer photomultiplier with complicated structure, which is far more expensive than the general small-sized photomultiplier. This raises system costs.    (2) The system disclosed in the U.S. Pat. No. 5,598,008 can be manufactured at a lower cost. By contrast, a problem of big attenuating of incident light is caused by a great amount of leakage from the aperture for scanning of stimulating lays, and a great number of reflections inside the light-collecting device to reach the photo-detector. When a system is designed, the reading time must be made longer in order to get the SN ratio of image.Further, according to this patent, the light-collecting device is installed close to said sheet. When the system is designed, flexibility is reduced in the layout of the light-collecting device. Designing may be difficult, depending on the type of said reading system.    (3) When the light-collecting efficiency is low in the image information reading system (scanner) such as X-ray film having a laser scanning system, reading time is prolonged to improve the SN ratio of image. The SN ratio of image is reduced if the reading time is reduced.